Melting of glass entails the combustion of large amounts of fuel in a melting furnace in order to provide the required melting temperatures by direct heating. The fuel (usually natural gas and sometimes fuel oil) is usually mixed with an excess of air beyond that theoretically required for complete combustion in order to assure that complete combustion actually occurs within the furnace for the sake of thermal efficiency, and particularly in the case of flat glass melting operations, to assure that oxidizing conditions are maintained within the furnace. This combination of conditions within a glass furnace is conducive to the oxidation of nitrogen in the combustion air to NO.sub.x.
NO.sub.x is a shorthand designation for oxides of nitrogen, such as NO and NO.sub.2. In the high temperature conditions of a glass melting furnace, the oxide and nitrogen formed is almost entirely NO, but after exhaust containing NO is released to the atmosphere, much of the NO is converted to NO.sub.2. NO.sub.2 is considered an objectionable air pollutant; it is also believed to be involved in the chemistry of smog formation. Therefore, large volume combustion sources such as glass melting furnaces are susceptible to governmental regulation that may severely restrict their operation.
Many proposals have been made for controlling NO.sub.x emissions from boilers, internal combustion engines, and the like, but most are incompatible with process furnaces as employed for melting glass. Many of the previous proposals involve catalytic destruction of NO.sub.x, but catalytic treatment of glass furnace emissions has been found to be unsatisfactory because the required catalyst contact devices quickly become plugged and corroded due to the particulate content and corrosiveness of glass furnace exhaust. Other proposals involve modifying combustion conditions, but substantial modifications in a glass melting furnace are restricted by the requirements of the melting process. Some NO.sub.x control proposals involve treating the exhaust gas within narrow temperature ranges, but in a glass furnace employing regenerators, wherein the firing is reversed periodically, the exhaust gas temperatures are continually changing. Yet another category of prior art NO.sub.x removal processes entails chemically reacting the NO.sub.x at reduced temperature, usually in a liquid phase. Such techniques appear to be prohibitively costly for application to glass furnace emissions due to the large cooling capacity and chemical consumption requirements and liquid waste disposal problems.
Non-catalytic processes for reducing NO.sub.x emissions generally involve injecting ammonia into an exhaust gas stream to selectively reduce NO to nitrogen and water, e.g. such as disclosed in U.S. Pat. No. 3,900,554. The major drawback with this approach is that the process is effective in only a narrow temperature range. U.S. Pat. No. 4,328,020 issued to Hughes, teaches that suitable conditions for ammonia reduction of NO.sub.x exist, or can be created, for a substantial portion of each firing cycle in a flue connecting primary and secondary regenerator chambers. Ammonia injection is discontinued whenever the temperature of the exhaust gas passing through the flue falls outside of the range of 870.degree. C. to 1090.degree. C. (700.degree. C. to 1090.degree. C. if the ammonia is injected in combination with a suitable amount of hydrogen). In another embodiment disclosed in the aforesaid patent, ammonia is sequentially injected into two or more zones of the regenerator as the temperature in each zone passes through the effective NO.sub.x reduction range. Although the methods taught by the Hughes patent are capable of removing a large portion of the NO.sub.x from glass furnace exhausts, the overall effectiveness of the ammonia reduction technique is reduced by the ineffectiveness of this technique during substantial portions of each firing cycle when the exhaust gas temperatures are unsuitable, i.e. outside of the above-delineated effective ammonia reduction temperature range.
A further non-catalytic chemical reduction of NO.sub.x technique is taught in U.S. Pat. No. 3,867,507 issued to Myerson. The Myerson patent teaches the chemical reduction of NO.sub.x in a combustion effluent stream by hydrocarbons and oxygen at elevated temperatures. This technique suffers the same disadvantages as those previously assigned to the Hughes patent ammonia injection technique, with the further disadvantage of the increased costs and operation difficulties of the additional oxidizing step. These non-catalytic processes also all have the additional disadvantage of being difficult to control.
Another presently available technique for reducing NO.sub.x contaminants in furnace exhaust products is taught in U.S. Pat. No. 3,890,084 issued to Voorhies et al. The Voorhies et al. patent teaches firing a lower bank of burners of a boiler with insufficient air (i.e. rich) and an upper burner bank with additional air (i.e. lean) to compensate for the air deficiency of the lower burner bank. The additional air from the upper burner bank is intermixed with the uncombusted fuel from the lower burners and such fuel is burned at a later stage downstream of the burners. This results in incomplete combustion and a reducing atmosphere (i.e. excess fuel) which is later oxidized to complete the combustion, which procedure is unacceptable for melting glass. The Voorhies et al. method is not suitable for use in a glass melting furnace, because in a glass melting furnace an oxidizing atmosphere must always be present to ensure proper glass quality. If excess fuel is present at the surface of the melting glass, a condition known as a reducing atmosphere, the excess fuel will discolor and produce brownish streaks in the glass.
U.S. Pat. No. 4,372,770 issued to Krumwiede et al. teaches a method of melting glass with a two-stage NO.sub.x control. The Krumwiede et al. patent teaches afterburning in conjunction with ammonia injection to achieve high levels of NO.sub.x reduction in glass melting furnaces. During portions of each firing cycle when thermal conditions render ammonia injection ineffective, fuel is injected into selected portions of the exhaust passages to suppress NO.sub.x formation. A limitation of this technique is that the heat generated by afterburning is not available to assist in the melting of the glass in the furnace melting chamber.
It would be desirable to have a method and apparatus for reducing furnace NO.sub.x emission levels which eliminates the disadvantages and shortcomings of existing techniques.